Forecasting the Weather and Storms 265 



the air, which allows a free escape of 

 heat from the earth by radiation at 

 night. As in the case of cold waves, 

 warnings are widely distributed in ad- 

 vance of the high that may cause frosts, 

 with great profit to the growers of tender 

 fruits and vegetables. 



In a general way the degree of cold 

 in a cold wave, or rather the departure 

 of the temperature from the normal of 

 the season, will be proportional to the 

 height of the barometer, and a neces- 

 sary concomitant of a cold wave is an 

 area of low pressure immediately in ad- 

 vance of the high pressure, the upward 

 movement in one increasing the down- 

 ward motion of the other ; and the 

 greater the difference in the barometer 

 between the two the greater the velocity 

 with which the air will gyrate about 

 and into the low, and the greater the 

 downward and outward movement of 

 the air in the high, and the more intense 

 the cold. It therefore follows that a 

 high that is not preceded by an active 

 low will have a less degree of cold for 

 a given pressure, and that the extent 

 and intensity of cold waves depends con- 

 siderably on the form and the charac- 

 teristics of the preceding low and its 

 location ; if north of the center of the 

 country the cold that follows will not 

 reach the Gulf states in severe form, if 

 at all ; but if a low of considerable 

 energy forms in the region of Texas 

 and moves northeastward to the Atlantic 

 coast, as nearly all lows do that orig- 

 inate in this region, and a high of equal 

 intensity develops at the same time over 

 the northern plateau of the Rocky Moun- 

 tains, the latter will be drawn far to the 

 south as the former moves out of the 

 way toward the east, and cold north- 

 west winds, driven by the high and at- 

 tracted by the low, flow into the Gulf of 

 Mexico itself, even reaching the islands 

 of the West Indies. 



It would be impossible for a cold wave 

 to come upon the Pacific Coast states 

 with the highs that drift in from the 



ocean, because of the warming effect of 

 the water upon the air to considerable 

 elevations ; but frosts and cold waves 

 visit the interior valleys of California 

 and other coast states and reach almost 

 to the ocean's edge. They are due to 

 highs that move southward and then 

 eastward along the plateau. The highs 

 may be moving eastward very slowly, 

 but the diameter of the areas covered by 

 them may increase so rapidly that some 

 cold air is pushed over the mountain 

 tops and flows from the northeast into 

 the interior valleys of the coast states. 



The U. S. Weather Bureau has 

 adopted certain arbitrary thermal limits 

 to determine what constitutes a cold 

 wave. Both the extent of the fall of 

 temperature and the degree of cold that 

 must be reached vary for season and 

 place. For example, in December, Jan- 

 uary, and February a cold wave in the 

 northern Rocky Mountain region occurs 

 when the temperature falls 20 degrees 

 in 24 hours and reaches a minimum of 

 zero or lower ; in Tennessee a fall of 

 20 degrees, and to 20 degrees or lower 

 is required, while along the Gulf coast 

 a fall of but 16 degrees and to 32 degrees 

 constitutes a cold wave. The fall in 

 temperature is reckoned from any given 

 hour of one day to the same hour of 

 the next day or from the minimum of 

 one day to the minimum of the next. 



The area and the intensity of cold 

 waves depend upon the size of conti- 

 nents and their distance from the trop- 

 ics. The interior of North America 

 and of Siberia have geographic condi- 

 tions that cause the most severe cold 

 waves of any parts of the world. If 

 the elevation of the Rocky Mountain 

 plateau in North America were one-half 

 of what it is and if the mountain chains 

 were leveled away, or even trended to 

 the east and west instead of north and 

 south, the vaporous atmosphere of the 

 Pacific, which extends upward but a 

 very short distance and which decreases 

 in density rapidly with elevation, be- 



